In the digital recording art it is common to record data signals in what is commonly known as the non-return-to-zero (NRZ) format. In the NRZ format there are two levels states, one representing a binary one (or "mark"), and the other representing a binary zero (or "space"). A bit pulse remains in one of its two level states for the entire bit interval. Clocking information from such data signals is generally recovered by using a one-shot (monostable multivibrator) to generate a pulse for every input data transition, the pulse being adjusted to be approximately one-half the bit width. The pulses are used to drive a phase-locked loop having a local oscillator tuned to the expected bit rate. The phase-locked loop can be thought of as a relatively narrowband high-Q filter whose center frequency follows or tracks the rate of the received bit transition pulses. The phase-locked loop generally includes some sort of phase detecting means to allow the local oscillator to follow both the phase and the frequency of the incoming data signal.
In a typical phase detector, the phase of the half-bit width pulse is compared with that of a square wave (and complement) produced by the local oscillator (usually a voltage controlled oscillator).
If the voltage controlled oscillator (VCO) is at the exact frequency and correct phase, then the mean voltage level out of the phase detector will be zero and will provide no error signal for the VCO. If, on the other hand, a frequency or phase error is present, the output of the phase detector will be asymmetrical, resulting in a DC level being generated by the phase detector. This signal is then filtered by the loop filter of the phase-locked loop, and is used to control the VCO to bring it back to the correct frequency and phase.
One common type of phase detector is known as the early gate-late gate phase detector and consists in its simplest form of two logic AND gates, one being used to AND gate the VCO waveform with the leading edge of the one-shot square wave, while the other is used to AND gate the 180.degree. phase-shifted VCO output with the trailing edge of the one-shot multivibrator complement. The outputs of the two AND gates are OR gated (summed) together.
However it is a characteristic of most such phase detectors and phase-locked loops that the phase of the VCO must be less than +90.degree. or greater than -90.degree. in order for the system to be pulled into synchronization. A phase difference between the VCO phase detector of less than -90.degree. or greater than +90.degree. tends to produce an error signal which will make the phase error even greater between the VCO and the incoming NRZ data envelope.
A further disadvantage is that the pulse width and frequency of a one-shot multivibrator is dependent upon the value of a timing element, namely a capacitor. In many data recordng systems, it is common to encounter data recording rates which span a 1000 to 1 range. This results in undue complexity for prior art bit synchronizers of the one-shot type since a different timing element or capacitor is needed for each anticipated data rate.